With more than $680 million in Iowa Department of Transportation construction projects in the works this construction season, detours have popped up all over the state. For most, driving a few miles out of the way for a while isn’t a big deal. But in some parts of rural Iowa where there aren’t as many detour options, the inconveniences can make a big impact on time, money, and frustration for travelers.
Weighing the options
When the team in District 5 determined that the bridge on U.S. 34 over White Breast Creek near Lucas needed to be replaced, they considered two alternatives.
Both options would produce a bridge that would handle the vehicle traffic and flow of the creek for years to come.
Impact to travelers is determining factor
The tipping point in the decision-making process came when the bridge designers considered how much the construction would cost the bridge user in terms of time, money, and hassle.
Using the ABC techniques, the bridge would be closed and traffic routed 37.4 miles out of the way. Using a formula that includes the number of vehicles expected to travel this route, the cost to these motorists was calculated to be $441,000 over the three-week duration of the detour.
The traditional construction using the temporary bridge to keep traffic moving would have very little impact to users.
“That user cost for the ABC option was simply too much for us to put on the people who need this rural road,” said Steve Seivert, a transportation engineer in the Iowa DOT’s Office of Bridges & Structures. “We needed to find a solution that would minimize disruption. We found that using the temporary bridge.”
Finding the right temporary solution
One of the options considered for the on-site run-around was a temporary bridge the Iowa DOT already owns and has used successfully for short-term, low traffic flow bridge needs. Seivert said, “When we took a look at the specifications of our temporary bridge, it was too small to work on the U.S. 34 project, but it stirred up the idea to find another ‘temporary’ option.”
The solution came in the form of an on-site detour bridge that was constructed with rented, prefabricated bridge components. A foundation built by the contractor specifically for the temporary crossing supported the structure.
The on-site detour utilizing rented bridge elements eliminates a long detour for travelers as a bridge is replaced on U.S. 34 in Lucas County. (Photo provide by Acrow)
The theory behind using prefabricated bridge parts to construct temporary structures dates back to WWII when parts were shipped and stored overseas to repair bridges damaged by the enemy.
For modern uses, think about a giant erector set. The bridge parts and pieces are all prefabricated to current design standards and the structures are typically held together with bolts and pins and not welded, which makes them easier to disassemble once they are no longer needed.
(Photo provided by Acrow)
For the U.S. 34 bridge, Seivert said, “We developed a special provision with specifications that called for the contractor to build the bridgefoundation or substructure of the bridge and then to use one of three suppliers to rent the decking and other bridge elements above the substructure. The contractor is responsible for the rental costs of the on-site detour bridge, so to help control their costs for rental they have an incentive to complete the new bridge quickly.”
The project, expected to be completed this fall, is going well and Seivert says it’s been a good learning experience for all involved. “Not only will we take this experience forward and use it as another option to minimize user costs, but this type of bridge can be constructed relatively quickly and could serve as a way to maintain traffic in an emergency situation,” said Seivert. “We’ll have data on what worked and what could be done differently that we can tap into in the future.”